1. Field of the Invention
The present invention relates to a battery can-forming plate; a battery can-forming method using the battery can-forming plate; and a primary battery and a secondary battery formed by the battery can-forming method. More particularly, the present invention relates to a battery can having a rough inner surface and a mirror-like outer surface.
2. Description of Related Art
Batteries are used as the power source of the various kinds of cordless equipments such as personal lap-top computers, cellular telephones, and the like which are widely used in recent years. In the portable equipments, the space occupied by batteries is very large. In order to make the portable equipments compact and light, it is necessary to provide them with batteries which are compact and light, and yet have a high performance. Thus, battery cans are required to have a high capacity.
Generally, nickel-plated steel plates are used as the battery can. The following two methods, namely, transfer drawing method and drawing & ironing (hereinafter referred to as DI) method are used to form a cylindrical battery can, using the nickel-plated steel plate. In transfer drawing method, a circular blank punched from a nickel-plated steel plate is fed to a plurality of dies having different drawing diameters to draw the circular blank so as to form a cylindrical steel plate, as described in Examined Japanese Patent Publication No. 7-99686. In DI method, a steel plate was sequentially passed through dies which have different drawing diameters and are coaxially vertically arranged, with a punch applying pressure to the steel plate to obtain a cylindrical steel plate.
A conventional nickel-plated layer of a conventional battery can-forming plate has a low hardness. Thus, when a battery can is formed by the press drawing method, the plated layer follows the plasticity of the steel in drawing the steel plate, with the result that the inner surface of the side wall (peripheral wall) of the battery can is smooth. In order to allow the inner surface of the side wall of the battery to make a close contact with a positive pole substance and reduce the contact electric resistance of the inner surface, it is preferable that the inner surface of the battery can is rough and slightly crackled. However, because the inner surface of the battery can formed by press drawing method is smooth, the inner surface thereof does not contact the positive pole substance closely, thus having a great contact electric resistance.
In order to solve the above-described problems, the present applicant proposed a method of causing a plated layer to crack at the time of the press drawing operation by forming a plated layer having a high hardness. However, in this method, after the steel plate is nickel-plated, it is annealed and rolled for tempering, and then, a plated layer having a high hardness is formed on the side of the steel plate corresponding to the inner surface of the side wall of the battery can. That is, this method requires a large number of processes. Thus, this method causes the manufacturing cost to be high.